The cool thing about these 4600 and 5700 motors is that RPM and Torque is almost independent now. There is more then enough RPM and Torque to gear it anyway you like. Of course, I'm not talking about the extremes, like a 78T with 30T pinion
Anything from 18 to 21T pinion would work fine in the B4, it doesn't really matter anyways, unless you're running on a large track with long straightaways, you'd be hardpressed to open up your throttle fully!
I run my B44 on a medium-sized track, and I use full throttle on my ride about 10% of the time, rest of the time, I'm throttling 20 to 40%, there's just too much power in these brushless!